The invention relates to the selective removal of dissolved acids or bases from aqueous solutions and their subsequent recovery in concentrated form. This is achieved by a combination of liquid membrane extraction using a selective reagent and by bipolar membrane electrolysis of the spent selective reagent.
It is known in the art that liquid membrane emulsions may be used to remove dissolved substances from aqueous solutions; see for example, U.S. Pat. Nos. 3,617,546; 3,637,488; and 3,779,907. The emulsion is characterized as having a dispersed or internal phase suspended in a continuous phase. The continuous phase of the emulsion is immiscible with the aqueous solution but is permeable to the dissolved substance. The dispersed phase, which usually is miscible with the aqueous phase, contains a reagent which cannot permeate through the continuous phase, but is capable of reacting with the dissolved substance when it reaches the dispersed phase to form a product which cannot permeate back through the continuous phase. The emulsion is contacted with the aqueous solution, whereupon the dissolved substance permeates through the continuous phase into the dispersed phase and is retained therein.
One way to achieve this conversion of the dissolved substance, after it has permeated into the dispersed phase, into a form in which it is incapable of permeating back through the immiscible continuous phase (i.e. the liquid membrane), is by neutralization. Another method is to form a precipitate. Thus, if the dissolved substance is phenol, which is permeable in its undissociated form through an oil liquid membrane phase, it can be extracted from a dilute aqueous solution using aqueous caustic encapsulated in a water-in-oil liquid membrane emulsion. When the phenol, after permeating through the oil membrane, reaches the dispersed caustic droplets, it is neutralized by the caustic, forming sodium phenate. The phenate ion is insoluble in oil, and, therefore, cannnot permeate back into the external dilute aqueous solution. Thus, by neutralization of the weak acid, phenol, it can be removed from a dilute aqueous solution by means of a liquid membrane emulsion containing a base.
However, it is clear that continued removal of this acid is limited by the amount of base present in the liquid membrane emulsion. As the base becomes consumed (by neutralization), the concentration of undissociated phenol in the caustic solution increases, resulting in less than complete removal of the phenol from the dilute aqueous solution. The reason for this is that the driving force for the permeation of the phenol through the liquid membrane is determined by the difference in concentrations of undissociated phenol in the dilute aqueous solution and the caustic solution. Once that difference shrinks, the motivation for the phenol to permeate into the liquid membrane emulsion disappears.
Consequently, removal of the spent caustic solution, and replacement of the same with fresh caustic in the liquid membrane emusion becomes an essential part of the process. In the past, this has been done in a number of ways, first breaking the emulsion by centrifugal, chemical or electrostatic means, followed by replacing the spent aqueous layer with fresh caustic, and disposing of the spent caustic by a number of alternates. One of these alternates is discarding, with its obvious detrimental environmental effects, since phenol is a toxic material. Fortunately, the volume of spent caustic is several orders of magnitude less than that of the original dilute solution, reducing the cost and inconvenience of the disposal step.
A second method is recovery of the phenol from the spent caustic by treatment with a strong acid, such as sulfuric acid, which will liberate the phenol, leaving behind a solution of sodium sulfate, which is much easier to dispose of than sodium phenate. While this method recovers phenol as a salable product, it requires the use of an additional reagent, e.g. sulfuric acid. Consequently, for each mole of phenol removed, at least one mole of caustic and one half-mole of sulfuric acid are consumed, and returned to the environment as undesirable sodium sulfate. In practice, incomplete utilization of the caustic in the phenol extraction necessitate the use of greater than stoichiometric amounts of both caustic and sulfuric acid.
Another method, and the subject of the present invention, is the splitting up of the sodium phenate salt formed into (1) phenol, which is separated, purified and used as a chemical, and into (2) caustic solution, which is recycled to the phenol removal step by reemulsification to form regenerated liquid membrane emulsion. A preferred method of achieving this splitting up is by the use of bipolar membrane electrolysis, allowing the sodium phenate feed solution recovered from the spent liquid membrane emulsion to be converted into one stream which is rich in caustic, and another stream rich in phenol.
It should be noted that since the stream rich in caustic generated by this technique is reused as recycled regenerated liquid membrane emulsion, it is not necessary to achieve a high degree of purity of the regenerated caustic. Thus, the regenerated caustic can still contain some phenol, provided that there is sufficient unreacted caustic in this stream to make emulsification and reuse worthwhile. By the same token, the regenerated phenol solution need not be entirely free of caustic, since the free phenol will be removed from this solution first by phase separation, followed by steam stripping. Since the presence of excess phenol in this solution ensures that any caustic present will be in the form of sodium phenate, caustic will not go into the liquid phenol phase during the phase separation, or into the vapor phase during the steam stripping step. Splitting a salt into its components by bipolar membrane electrolysis is simplified considerably if the resultant acid and base products need not be pure. Only by combining the bipolar membrane electrolysis with a membrane separation scheme, such as liquid membrane emulsion extraction, can this advantage be realized.
Another advantage of this technique, the combined liquid membrane emulsion extraction and bipolar membrane elecrolysis of the spent internal aqueous phase of the emulsion, is that liquid membrane extraction exhibits selectivity between different dilute acids or bases, allowing separate removal and recovery of different contaminants or solutes present in the dilute aqueous feed solution. Simple direct electrolysis of the dilute feed liquor could not achieve this separation.
Another advantage of this technique is that dilute solutions can be converted into concentrated products, since the liquid membrane extraction step per se is a concentration step, vastly increasing the concentration of the acid or base in the incoming feed. Direct bipolar membrane electrolysis would not achieve such a concentration. Further, electrolysis of a dilute solution has to overcome large resistance losses on account of the low conductivity of dilute aqueous solutions, a problem neatly avoided by the present technique, which electrolyzes a concentrated (e.g., spent caustic) solution.
A further advantage of the present technique is that it allows the use of internal reagents in the liquid membrane system which cannot be considered for environmental or economic reasons in a throw-away situation, such as simple discarding of the spent internal phase of the used emusion. Also, reagents can be used which have better solubility properties than the reagent generally used in a throw-away situation. The use of potassium hydroxide is a case in point, compared to the more usual caustic soda, or, in the case of amine removal, use of hydrochloric rather than sulfuric acid.
A further advantage is that the new combination permits the use of more environmentally acceptable, although more expensive reagents in the internal phase of the emulsion. In conventional liquid membrane treating, a small quantity of the internal phase will leak out into the aqueous phase being treated, resulting in minuscule, yet measurable contamination of the aqueous treating effluent. On the other hand, since the spent reagent has to be discarded, economics usually dictate the use of the lowest cost reagent. Reagent cost and effluent contamination considerations do not necessarily point to the same reagent. With the use of the present invention, cost considerations for the used reagent can be disregarded, making the choice of an environmentally least objectionable reagent so much easier.
Other advantages of the present invention will become apparent in the following description.
In conventional liquid membrane (LM) extraction of, say, a dilute weakly dissociated acid such as phenol, which is somewhat oil soluble, the aqueous liquid feed containing said acid is contacted in an agitated vessel with dispersed globules of a water-in-oil emulsion, where the continuous (oil) phase of the emulsion contains a surfactant and possibly an acid solubilizer, and the encapsulated dispersed aqueous phase of the emulsion contains a base which can neutralize any permeated acid. The undissociated acid will keep on permeating through the oil film into the internal droplets, where the base will convert the undissociated acid into an oil-insoluble salt. This effectively prevents return of the extracted acid into the outside bulk aqueous phase, resulting in accumulation and concentration of the extracted material, albeit in changed form, in the internal droplets. This removal will continue as long as free base remains. The equilibrium governing the removal of a dilute acid or base, e.g. phenol, from an aqueous solution into the concentrated internal reagent of a water-in-oil liquid membrane emulsion is described in the literature, such as by R. P. Cahn and N. N. Li, in "Separation of Phenol from Waste Water by the Liquid Membrane Technique", Separation Science 9(6), pp. 505-519, 1974.
After the extraction has proceeded to the desired extent, the emulsion is allowed to settle away from the treated feed solution which is discarded or subjected to further treatment. The emulsion is broken, using one or a combination of several means of separation, such as centrifugation, agitation, addition of chemicals, or electrostatic coalescence. The preferred method of separating the emulsion phases is by means of an electrostatic coalescer. The internal aqueous phase broken out of the emulsion is further treated as described hereinbelow, while the recovered oil phase is recycled to emulsion making and thus reused directly in the process.
The spent internal reagent solution, comprising salt and left-over (excess) reagent, e.g. base, is now fed to the bipolar membrane electrolysis for recovery of the removed acid in concentrate form, and reforming of the base, allowing its recycle to the extraction process after incorporation into the emulsion. The heart of the bipolar membrane electrolysis is the bipolar membrane, which consists of an anion-permeable anion-exchange resin layer, and a cation-permeable cation-exchange resin layer, sandwiched together. The membrane is also somewhat water permeable. When such a membrane is immersed into an aqueous solution, and a potential is applied across it, water at the interface between the two layers will be split into protons and hydroxyl ions. The protons will migrate through the cation-permeable side (which must be the side closer to the cathode of the applied field) and the hydroxyl ions will drift to the anode via the anion-permeable layer. The cell compartment adjacent to the cation-permeable layer will become enriched in protons, thus turning acidic, while the compartment adjacent to the anion-permeable layer will become enriched in hydroxyl ions, i.e. it will turn alkaline. By a judicious combination of bipolar membranes, cation-permeable membranes and anion-permeable membranes, to be described hereinbelow, it is possible to build up a cell stack, so that when the spent internal aqueous solution is allowed to flow through the appropriate compartments of such a stack, and water through others, and an electric potential is applied across the cell stack, the salt in the spent aqueous solution is electrolytically decomposed into its constituent acid and base, without the evolution of hydrogen, oxygen, or other undesirable by-products of conventional electrolysis. For this reason, bipolar electrolysis requires a much lower voltage per unit cell than conventional electrolysis, about half the voltage or less, with a corresponding saving in electric power consumption.
It is a special advantage of bipolar membrane technology that the spent reagent is regenerated in a form and in a concentration which is sufficiently dilute so that it can be used directly in the reemulsification step for recycle of "regenerated" liquid membrane emulsion. This is frequently not true in conventional electrolysis, which often regenerates either the acid or the base not in its recyclable form, if at all, but in a form which requires considerable further reaction and processing before it can be recycled.
The dissolved substance, which was removed from the dilute aqueous solution feed by means of the reagent-loaded liquid membrane emulsion, is regenerated in the bipolar membrane cell, separated from the reagent, as a much more concentrated aqueous solution than in the original feed. The regenerated dissolved substance, as a result of the separation achieved by the selective liquid membrane extraction, will also be free of other contaminants which accompanied it in the dilute aqueous feed, and which frequently interfere with any attempts to recover it directly from the dilute feed. From the said more concentrated aqueous solution, the dissolved substance can be recovered economically by any number of conventional means such as phase separation, extraction, distillation, steam stripping, etc.
This process is eminently suited to the recovery of dilute acids and bases from aqueous solutions in which they are admixed with other materials, frequently other acids and bases, from which they can be separated by the emulsion liquid membrane technique. Thus, if it is desirable to remove phenol, a slightly acidic compound, from its dilute aqueous solution also containing other acids or their anions, such as sulfuric acid or sulfate ion, chloride, nitrate, phosphate, etc., a preliminary emulsion liquid membrane separation using an aqueous caustic-containing emulsion will pull the phenol selectively out of this dilute solution. The other anions and their respective acids, which are all essentially oil-insoluble, will not permeate into the internal droplets of the emulsion through the oil membrane, effecting a very selective separation of the phenol from the other acids in the original feed. Demulsification of the spent emulsion and bipolar membrane electrolysis of the spent internal aqueous phase of the emulsion will liberate the extracted phenol and regenerate the spent caustic for reuse. Since the concentration of phenol in the spent emulsion can be very much higher than its concentration in the incoming feed, recovery of the phenol as a product is much simpler than if this recovery had been attempted on the incoming feed. Another feature of the present invention, which is demonstrated by this example, is that if bipolar electrolysis of the incoming feed were attempted, the recovered acid would not be just phenol, but would also include sulfuric, hydrochloric, phosphoric and other acids present as such or as their anions in the incoming feed. Not only would this result in a highly contaminated product, requiring further purification, but it would also involve the expenditure of unnecessary quantities of power to regenerate the undesired contaminant acids, e.g. the sulfuric, hyudrochloric and phosphoric acids.
While the separation of phenol from sulfate or chloride is inherent in the differences in oil solubility, and consequently oil permeability of phenol vs. sulfuric, hydrochloric and other inorganic acids, such differences in solubility and permeability can be artificially induced in the system by addition of solubilizers to the liquid membrane, i.e. to the continuous oil phase of the liquid membrane emulsion. Such solubilizers may be oil-soluble clathrating, complexing, chelating or ion exchange materials which may be selective to one or several cations or anions in the aqueous feed. In the case of cations, such materials may be DHPA, di(ethylhexyl) phosphoric acid or a chelating agents for copper, for example LIX64N.TM., a material supplied by General Mills Chemical Co, and for anions Amberlite LA-2.TM., an oil-soluble secondary amine manufactured by Rohm & Haas, or other materials which provide the requisite solubility and selectivity to the membrane phase.
In general, most inorganic acids and, of course their anions, are essentially insoluble in the undoped membrane phase, and are consequently prevented from being picked up by the liquid membrane emulsion. On the other hand, organic acids, such as acetic, propionic, butyric, benzoic, and other naphthenic or aromatic acids, phenol etc. are sufficiently soluble in oil to give some permeability to the undissociated form (not the ionic form) of the acid. A few inorganic acids, or their anhydrides, e.g. hydrogen sulfide, carbon dioxide, or sulfur dioxide, have some oil solubility and, therefore, permit selective removal of these acids in the presence of strong acid contaminant such as sulfuric and hydrochloric acids.
The same can be said for cations or their corresponding bases. Alkali and alkaline earth metal ions and their corresponding bases are not soluble in the oil membrane without appropriate doping. On the other hand, ammonia, amines and other basic organic compounds are soluble in oil, and the membrane phase is therefore permeable to these materials, allowing their selective removal from dilute solutions also containing other cationic contaminants like sodium or calcium ions. Just as previously discussed in the case of phenol removal, amines or ammonia can therefore be selectively removed and recovered when present in dilute concentration in a feed also comprising oil-insoluble cations like sodium, calcium, ferric, etc. Attempts to recover the ammonia or amine directly from the feed by bipolar electrolysis would result in wasteful expenditure of electrical power and the formation of a mixture of dilute ammonia or amine together with sodium and calcium hydroxide and other undesirable cationic contaminants. It is, of course, obvious that attempts to remove the ammonia or amines selectively by liquid membrane treatment alone would be successful, but the end product would not be free ammonia or the amine, but the ammonium or amine salt of the acid which was used in the liquid membrane emulsion fed to the treater, and the process would consume acid commensurate with the amount of ammonia or amine removed.